1. Field of the Invention
The present invention relates to an image forming apparatus which has a fixing member for heating, pressurizing, and fixing a toner image and can change the image forming direction for forming a toner image according to the temperature of the fixing member and a fixing device of the image forming apparatus.
2. Description of the Related Art
As a fixing device used in a copier of an electro-photographic method or a printer, there is a fixing device available for inserting a recording medium between fixing rollers composed of a heating roller and a pressure roller or fixing belts composed similarly and heating, pressurizing, and fixing a toner image.
In such a fixing device, like small-sized recording media or recording media to be conveyed in the vertically long direction (R direction), when recording media narrower than the width of the fixing device are continuously fed, at the end of the fixing device, heat is not transferred by paper feed and the surface temperature rises abnormally. When the fixing step of using the end of the fixing device is executed next in this state, image defects may be caused at the image end due to high-temperature offset or image quality defects may be caused in recording media due to shrinkage or crinkles. On the other hand, when recording media are continuously fed in the state that those media are conveyed in the horizontally long direction and the fixing device is used up to the end thereof, the end of the fixing device is lowered in temperature compared with the central part thereof due to radiation of heat to the outside and fixing defects may be caused due to an insufficient temperature.
Therefore, conventionally, in Japanese Patent Disclosure No. 6-186883, a fixing device for controlling heat generation of a heater from detected values of a temperature detection element at the central part of a heating roller and a temperature detection element at the end thereof and controlling the temperature of the heating roller is disclosed. The conventional fixing device detects the surface temperature of the heating roller to realize energy conservation. Furthermore, in Japanese Patent Disclosure No. 10-301436, a fixing device for detecting the surface temperature at the central part of a heating roller and the surface temperature at the end thereof, and when the surface temperature at the end is low, controlling a heater, and preventing image detects due to an insufficient temperature at the end is disclosed.
However, in the aforementioned fixing devices, the heater is controlled, thus the surface temperature of the heating roller is controlled. Therefore, when the heating roller abnormally rises in temperature or is insufficient in temperature, it forces feed of recording media to wait once until the heater is controlled and the temperature of the heating roller reaches the control temperature realizing satisfactory fixing. And, after waiting for feed of recording media, when the heating roller reaches the control temperature, the fixing operation must be restarted. Therefore, the image forming speed is decreased and the productivity is prevented from improvement.
Therefore, an image forming apparatus in which fixed images of high image quality can be obtained by a heating, pressurizing, and fixing device, and the image forming operation can be speeded up easily, and the image forming direction can be changed and a fixing device of the image forming apparatus are desired.